The present invention relates to a curvable tube which is adapted to be connected to the upper end of a flexible tube of an endoscope and which may be inserted into a body cavity. More particularly, the invention relates to the arrangement of such a curvable tube having a structure with no discontinuous curve at the connecting point of the flexible tube and the curvable tube.
The flexible tube of an endoscope, which is inserted into the body cavity, is connected to a curvable tube assembly and the end of the flexible tube is advanced along a path in the body cavity such as to locate a part to be exmained in the body cavity under observation. In general, the flexible tube is constructed by covering a coarsely wound elastic coil with a cover tube. The flexible tube thus formed suffers from a defect in that it is relatively stiff or curves unevenly against a bending action.
The curvable tube of the prior art is constituted by a number of articulation rings which are juxtaposed by pivotally supporting adjacent articulation rings at two points with the rings covered by a flexible sheath so as to have a swivel articulation function. The curvable tube may be bent in a preferred direction by operating pull strings at the manual operation unit of the endoscope.
In the conventional endoscope, the ends of finely wound non-compressive stay coils provided for the pull strings for bending the curvable tube are fastened to the connecting point of the flexible tube and the curvable tube and an annular relay tube is used to provide a uniform contact surface for the end of the flexible tube having a spiral structure and to connect the two tubes. The relay tube is unbendable. Therefore, when the curvable tube of the tube assembly is bent in the body cavity, because of the presence of the relay tube, a discontinuous curve occurs between the flexible tube and the curvable tube which are bent along the body wall.
Moreover, the conditions of the path in the body cavity along which the end of the tube assembly is passed to reach the part to be examined are not usually uniform. Sometimes the end of the tube assembly must pass along a path which, as in the sigmoid flexure of the large intestine, curves against the direction of insertion of the flexible tube as illustrated in FIG. 1.
When the above-described conventional tube assembly is passed along such a path in a body cavity, specific difficulties are commonly encountered. Specifically, no matter which way the curvable tube 2 is bent, in the range of from the state indicated by the solid line to the state indicated by the chain line in FIG. 1, in a greatly curved and narrow region of an insertion path in a body cavity such as the zigzag path 1 as shown in FIG. 1, it is impossible for the relay tube 3 which forms the discontinuous curve between the curvable tube 2 and the flexible tube 4 to pass smoothly through the region. Therefore, if the flexible tube is inserted under pressure into such a region, the region may be damaged.
The insertion path in a body cavity may include a portion which has a widely varying curvature although not so abrupt as that in the zigzag path shown in FIG. 1. A device in which the active length of a curvable tube can be changed has been proposed to provide a smoothly insertable tube assembly. See, for example, Japanese Published Patent Application No. 35794/1976. The curved portion corresponding to the active length of the curvable tube is uniform in curvature. Therefore, when the curvable tube with a shortened active length is inserted into a short curved path, both ends of the short path are expanded and, when the curvable tube with an elongated active length is inserted a long curved path, the end portion of the curvable tube is merely bent like a hook and cannot be extended satisfactorily along the insertion path. In order to overcome this difficulty, the conventional device is so designed that the front part of the relatively long curvable tube structure is freely bendable in four directions, that is, both horizontally and vertically, while the rear part is bendable only in two directions, that is only one of horizontally and vertically. For this purpose, the front part and the rear part of the curvable tube are made different from each other in the combination of articulation rings forming the curvable tube thereby to control the directions of bending of the swivel articulations.
Although the active length of the curvable tube of the device can be changed as described above, it is impossible for such a curvable tube to smoothly pass through a narrow and zigzag path of the sort shown in FIG. 1 because, even if the curvable tube is curved reversely with its active length shortened, that part is followed by the relatively long part which forms the rear part of the curvable tube and which is controlled to be unbendable.
In view of the above-described difficulties accompanying a conventional tube assembly of an endoscope, an object of the present invention is to provide a curvable tube assembly for an endoscope which can pass through not only insertion paths of different curvature in the body cavity but also narrow and zigzag insertion paths.